The present invention relates to the field of dental restorations and more particularly to the preparation of dental crowns with a system and method that easily separates contaminates from precious metal scrap generated in the making of the crowns and facilitates a high rate of recovery thereof. The present invention is also useful in the jewelry and like industries where noble and high noble metal items are made.
The most prevalent crown construction for dental restorations is the porcelain veneered cast metal crown and full contour metal crowns. The cast metal crown is fabricated using a relatively thin metal understructure formed from casting noble and high noble metals into an investment mold of a wax or plastic pattern of the metal understructure or full contour crown. Dental porcelain is then applied in layers over part or all of the understructure and fired at high temperature to form a veneer layer. The metal understructure may be formed from a base ally (non-precious) noble based metal or a noble based alloy with varying quantities of gold, platinum, palladium, silver, nickel, and an array of xe2x80x9ctrace elementsxe2x80x9d.
The crown typically is made in a process where a model or pattern of the patient""s teeth is made. The pattern of the crown is made of out of wax. The pattern is sprued, invested, burned out and a metal casting is made therefrom in the classic lost wax technique. Each metal unit is cut off from the base (or button) with a disk, typically made of carborundum. The button is saved and reused. The scrap, from grinding or cutting and typically collected by vacuum, is not readily reusable and is saved for processing at an off-site refiner. Unusable, contaminated scrap is generated at most or all the subsequent steps as well. For example, scrap is generated when the units are separated from the sprues. Scrap is generated when the inside and outside surfaces of the units are prepared with stones. The scrap contains at least contaminates from the stone cutting and surface preparation devices. Significantly, the expense of sending the scrap to be processed by an outside refiner yields a low rate of recovery of the value of the material, on the order of about sixty percent.
Similar disadvantages exist in the jewelry industry, where lost wax casting techniques, and the like, are employed to make small precious metal items.
It would be beneficial to provide an improved method and system for preparing crown understructures and full contour crowns that overcomes these disadvantages.
One aspect of the present invention provides a method for recovering noble and high noble metal from scrap generated during the manufacture of noble and high noble metal units including providing a cast metal unit. The unit is made of at least some noble metal. The surface of the unit is prepared with an abrading element. The preparing step further includes generating a magnetically attractive particulate contaminate and generating an amount of scrap thereby. The scrap includes noble metal and a percentage of particulate contaminate. The noble metal is magnetically separated from the contaminate.
Another aspect of the method of the present provides a casting button and sprue. The method further includes removing the button from the unit and removing the sprue from the unit with the abrading element before surface preparation of the unit, generating an additional amount of scrap thereby; and combining the generated amounts of scrap before magnetic separation thereof.
The unit may be a dental crown understructure. The unit may be a jewelry item. The abrading element may be a tungsten carbide bur operated at a speed of about 10,000 to about 300,000 RPM. The bur may be operated at about 40,000 RPM. The step of preparing the surface of the unit may take place in a substantially closed housing. The step of magnetically separating the noble metal may include transferring the scrap into a refining jar, wherein the refining jar comprises a magnet, and agitating the scrap therein.
Another aspect of the present invention provides a system for preparing noble metal units and recovering noble metal from scrap generated during the manufacture thereof, including at least one abrading element and means for operating the bur at a speed sufficient to prepare the surface of the units. A substantially closed collection unit is provided for operation of the at least one bur therein, for collecting scrap generated during preparation of the unit by the bur, wherein the scrap includes a percentage of magnetically attractive contaminate. A refining unit is provided including a magnet positioned therein for magnetically removing the tungsten carbide contaminate from the scrap.
Other aspects of the system of the present invention provide a collection unit that may include a housing, at least a portion of the housing being transparent. The housing may include a metallic portion. The housing may include left and right sides, at least one of which includes a tool port formed therein through which a tool may be inserted therethrough. Each of the left and right sides may include an opening into which a plate is rotatably positioned, each of the plates including an orifice formed therein through which a hand may be inserted therethrough. Each orifice may be eccentrically formed in each plate. The refining unit may include a glass portion and a lid portion, the lid portion being releasably positioned to close the glass portion. The magnet may be a rare earth magnet. The magnet may be affixed to the lid for positioning within the glass portion of the refining unit.
The invention provides the foregoing and other features, and the advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention and do not limit the scope of the invention, which is defined by the appended claims and equivalents thereof.